3G4 PALAEONTOLOGY OF ILLINOIS. 



lower part of the whorls, and marked with more or less strongly 

 denned, regular, oblique stria3 or costee near the suture above, 

 rarely passing down upon the middle and lower part of the 

 body whorl; sometimes covered with small nodes or granules. 



This genus, as we understand it, includes at least three rather marked sec- 

 tions, which may be defined as follows : 



1. NATICOPSIS, (proper.) 



Shell smoother, excepting oblique strife parallel to the lines of growth, near the 



suture. 

 iV. Phillipsi, McCoy,* JV. ventrica, Norwood and Pratten, Natica plicistria, 

 Phillips, etc. 



2. 



Shell with oblique, strong strise or costse, most distinct near the suture, but 



well defined over the whole body whorl. 

 Nerita subcostata, Goldf. ; Litlorina biserialis, Phillips, etc. 



3. TRACHTDOJMIIA, Meek and Woethen. (rpa^w;, rough; dw/ia, a house.) 



Shell with surface ornamented with small, more or less regularly disposed, 



nodes or granules. 

 Naticopsis nodosa, Meek and Worthen ; f N. (Trachyd.) Wheeleri=[Littorina 

 Wheeler, Swallow) ; Natica Marix, Murchison, deVern. and Keys.; Bucciuum 

 breve, Sowerby, etc. 



In redefining this genus at a later date, in his valuable work on the British 

 Palaeozoic Fossils, p. 301, Prof. McCoy describes it as having " a minute um- 

 bilicus, only seen in the cast." As we have had an opportunity to examine 

 numerous fine specimens of several species of this genus, without seeing any 

 traces of a perforation in the columella, we can but regard the minute um- 

 bilicus mentioned by Prof. McCoy in the cast, as merely the cavity left in it 

 by the columella itself. 



The shells of this genus often have so much the aspect of some types of the 

 Neritidaz, that we were at one time under the impression that they must belong 

 to that family. On breaking open a number of specimens, however, of various 

 species, and finding that the inner whorls are not absorbed away as in the Neri- 

 tidx, we were led to doubt the correctness of that conclusion, and since seeing 

 numerous specimens of opercula found associated with these shells in our Coal 

 Measures, under circumstances scarcely admitting of a doubt that they belong 

 to them, we have been confirmed in the opinion that this genus must belong to 

 some other family. The opercula alluded to, have exactly the form and size 

 corresponding to the aperture of typical species of Naticopsis, with which they 



* This being the first species mentioned by McCoy, and agreeing best with his diag- 

 nosis, may be regarded as the type of the genus. 

 f We regard this as the type of this subgenus. 



